Perception in biological terms can be defined as the registering of information or the detection of environmental changes by an organism. Sense organs are specialised receptors designed for detecting environmental status and change, both internal and external. They are the means whereby an animal perceives it's environment. Sense organs convert diverse stimuli from the environment into nerve impulses, the common language of the nervous system. Here is a survey of the sensory systems of various animal phyla, with emphasis on the most basic of these:
Sensory systems in the Vertebrates
Vertebrates are characterised by major structural and functional developments in the nervous system. They have a clear separation of the brain from the spinal chord, and the sense organs are developed well beyond those of the invertebrates. For example, vertebrates possess paired eyes with lenses and inverted retinas; pressure receptors, such as paired ears designed for equilibrium, which then evolved into sound receptors; and well developed chemical receptor organs for taste and smell, particularly in respect to often extremely sensitive olfactory organs.
Sensory systems in the Arthropods (insects, crustaceans, spiders)
Sensory organs are found in great variety in the arthropods, from the compound eye to simpler senses that have to do with touch, smell, hearing, balancing, chemical reception, temperature, humidity and others. In insects, airborne sounds and mechanical stimuli dealing with touch, pressure, vibration and so forth are detected by very sensitive hairs connected to nerve cells (sensilla).
The central nervous system and ‘brain’ of arthropods consists of masses of nerve cells which function as coordinating centres chiefly by inhibiting or limiting certain reflex actions, although learning is also significant in many species. The sensory systems of arthropods contribute not only to very simple behaviour patterns such as kineses (nondirectional increase in activity upon stimulus) and taxes (directional movement in response to a stimulus), but also to highly complex behaviours such as the intriguing waggle dance of the honey bee (described here).
Sensory systems in Flatworms
Flatworms are the most primitive animals to have bilateral symmetry. They include parasitic species such as tapeworms and flukes, as well as larger, free-living species. Their nervous system consists of a pair of anterior ganglia (the ‘brain’) with lengthwise running nerves connected by transverse nerves, forming a ladder-like pattern. There are a variety of sensory cells spread over the body connected to this nervous system. Many of these are chemoreceptors or tactile cells and are not greatly differentiated from the epidermal cells around them.
In free living flatworms, there are two groups of sensory cells known as auricular organs at the sides of the ‘head’ and these are though to be specialised taste receptors. Ocelli, or light-sensitive spots, are also common in these species. These ‘eyes’ allow the animal to discriminate between light intensities and determine the direction of illumination and orient in relation to it. Some free living species also have statocysts (equilibrium sensing cells) and receptors for sensing water current direction.
Here is some basic information on flatworms, and here is a photogallery of some large, free-living species.
Sensory systems in the Cnidaria (jellyfish, sea-anemones, corals)
Cnidarians come in many shapes and forms and are characterised by the lack of concentrated grouping of nerve cells that would suggest a central nervous system. Rather, they possess a diffuse nervous system made up of networks of neurons called ‘nerve nets’. Many of the neurons in nerve nets are also unusual in that they allow transmission of nerve impulses in both directions. Although without central control, the movements of Cnidarians are coordinated by simultaneous firing of nerves and contraction of muscles.
Sensory systems in the Cnidaria are simple. Sensory cells are scattered among the epidermal cells, especially around the mouth and tentacles. The free end of each sensory cell bears a flagellum, which is the sensory receptor for chemical and tactile stimuli. The other end branches into fine processes, which synapse with the nerve cells. Jellyfish have simple sense organs called rhopalium, which contain statocysts and pits lined with sensory epithelia, as well as ocelli in some species.
Cnidarians are also characterised by stinging organoids (nematocysts). It is interesting that an animal with such simple sensory apparatus can provoke such intense sensory reactions in other animals - as anyone who has been stung by a bluebottle or jellyfish would attest!
Here are some pictures of various Cnidaria.
Sensory systems in the Porifera (sponges)
Sponges can be regarded as loose aggregations of cells without definite form or symmetry. Their cells act more or less independently and show minimal cooperation or coordination. Sponges have negligible bodily movement and have not evolved a nervous system or sense organs. For this reason, examination of sensory systems at the next most primitive level is best undertaken by consideration of the single celled organism.
Sensory systems in the Protozoa and Bacteria (single celled organisms)
The primary way in which single celled organisms sense their environment is through chemoreception, in much the same way that humans smell. Bacteria and protozoa detect chemicals through receptor proteins in their membranes. Sensing of molecules leads to behavioural changes such as movement with or against the concentration gradient of a stimulus chemical.
Ingenious experiments have shown that some bacteria ‘know’ to keep moving in a certain direction along a concentration gradient not through the differences in concentration of stimulus molecules at the different ends of the organism, but through a primitive form of memory associated with the different rates of enzymatic reactions which ensue from receptor stimulation (read this article for more information). Other experiments have shown that the reactions of some paramecium (a type of protozoan) is dependent on electrical potential differences across the cell membrane, analogous to the way in which nerve impulses are conducted.
Chemoreception is not the only form of perception in the single celled organisms, although it is the most studied. Other sensory responses occur, in varying degrees amongst different species, to light, heat, gravity, mechanical contact and electric fields. Here is another article, which discusses the evolution and varieties of perception in single celled organisms.
Sensory systems in bacteria and protozoa can lead to quite complex behavioural responses, to the extent that HJ Jenning, a leader in the field in his day, said of the amoeba in 1906 that:
“The writer is thoroughly convinced, after long study of the behaviour of this organism, that if Amoeba were a large animal, so as to come within the everyday experience of human beings, its behaviour would at once call forth the attribution to it of states of pleasure and pain, of hunger, desire, and the like, on precisely the same basis as we attribute these things to the dog”.
Perception in other organisms
Plants respond to external stimuli though growth responses called tropisms. Tropisms include responses to light, gravity and touch (phototropism,gravitropism and thigmotropism respectively).
In the case of phototropism, the initial response to light which leads to the subsequent growth response arises through protein-pigment complexes within cell membranes. The mechanisms of thigmotropism are poorly understood and operate differently in different species. Some species are believed to react to touch through stimulation of hairs on epidermal cells affecting membrane ionic permeability, which thereby gives rise to an action potential. More on thigmotropism can be found here. Gravitropism arises through specialised cells containing gravity responsive starch grains.
Outside the scientific mainstream, some researchers claim to have detected emotional and telepathic-like responses in plants (click here for more info), though the validity of these experiments is widely contested.
Viruses are conventionally regarded as the most simple living things. They attach to their host cells through recognition and attachment of viral proteins to receptor sites on the surface of the host cell. Thus, viral perception can be regarded as a form of chemoreception.
What’s so special about the neuron?
At this juncture, it may appear as if there has been an unjustified conflation of the activity of neurons with that of other types of cellular and organismic activity. Therefore, it is worth considering whether there is anything that uniquely distinguishes neural activity from other cellular and molecular processes.
The key features that allow neurons to transmit nerve impulses are a permeable membrane and a potential difference caused by variations in concentrations of ions on different sides of the membrane. This potential difference results in the movement of ions across the membrane and the propagation of the electrochemical impulse along the neuron.
Selective membrane permeability is a feature of all cells. As mentioned above, membrane potential differences are implicated in such activities as the movement of plants in response to touch and the coordination of protozoan cilia and flagella movement (here is another interesting article on protozoan behaviour). The mechanisms of ciliary coordination through membrane potential polarisation and depolarisation have in fact been postulated as the evolutionary antecedent of neural transmission and coordination in the multicellular animals. Clearly then, there is no difference in kind between the activities of neurons and other cells.
Molecular perception
The next question that arises is whether there is any difference in kind between cellular reactions and those at the molecular level. The cell membrane obviously gives a boundary which allows coordination and integration of cellular activity, in much the same way that the brain allows integration and coordination of the human body.
However, again there does not appear to be any difference in kind between the reactions of a cell to it’s environment and the reaction of a molecule or atom to it’s neighbour. Both entail an energetic response to a stimulus. In fact, the reactions at the cellular level would be explained by most biologists as the result of reactions at the molecular level, with no admission of the emergence of a unique category of interaction (though this is not exactly my view). For instance, the attractive bonding of the hydrogen end of one water molecule to the oxygen end of another is analogous to the movement of ions across the membrane of a neuron which result in a nerve impulse.
Perception as Causation
The examination of perception at different levels could be continued into the subatomic and quantum particle realms. The picture which emerges is that of perception being synonymous with the interaction of any entity with another. Thus, perception has an intrinsic connection with causation. To perceive is to act and to be acted upon. For instance, the mutual attraction of two particles is the result of the mutual perception of each particle by the other. Or, at the macroscopic level, the movement of an iron filing toward a magnet is the perception of the magnet and the filing of each other.
This fusion of perception with causation is obviously an idea which requires further development (but not in this post). Such development along these or similar lines has been pursued by philosophers such as Whitehead and, more recently, Gregg Rosenberg, whose book on the connections between consciousness and causation has recently been published (click here).
Implications
Thus far, perception has primarily been considered in this post from an objective perspective - from the viewpoint of an outside, scientific observer. As such, it could be said that all that has been demonstrated is that perception occurs through the same physical and chemical processes that operate throughout nature. This is no more than a reductionist explanation of perception in physicochemical terms. However, now we turn to a consideration of perception in it’s subjective aspect - how it qualitatively feels from the ‘inside’ to sense the environment.
Reductionism works both ways. If we are to explain nature in physical terms, then such explanation should encompass the whole of nature, including the fact of the subjectively perceiving person. The sentient properties we feel in ourselves and assume to exist in other humans must be explainable in the same terms as other physical systems. It has been shown that perception, considered from the objective viewpoint, is ubiquitous in nature and there is no particular point in the evolutionary chain at which it suddenly appears from nowhere. Therefore, we are led to ask what reason is there for not also attributing a universality to the subjective aspect of perception?
The extension of human qualities to other natural systems is usually dismissed as naïve anthropomorphism. But, it is worth considering whether the anthropomorphic approach is totally unwarranted. If we seek to explain human qualities as natural properties, then these qualities must surely be a property of nature. As Whitehead phrased the issue:
“Any doctrine which refuses to place human experience outside nature, must find in descriptions of human experience factors which also enter into the descriptions of less specialised natural occurrences. If there be no such factors, then the doctrine of human experience as a fact of nature is mere bluff.”
There is no obvious reason why the attribution of subjectivity should not be extended beyond human and similar organisms.
At this point, it may be objected that human perception is distinctly a property of the human brain and that no analogy between the human brain and other natural processes has been demonstrated. The present neurobiological, if not philosophical consensus, is that subjective perception occurs through the brain constructing or representing models of the world - it is not the eyes or ears which perceive, but the brain.
However, subscribing to this view does not alter the conclusions arrived at here. The brain utilises the same physical and chemical processes as the rest of nature. Leaving aside the difficult issue of how multifarious neuronal events are bound into a single unified subjective experience, the representationalist framework asserts that perception arises through the transmission of impulses between neurons in the brain. But the binding of a neurotransmitter to a postsynaptic membrane receptor is another form of perception, analogous to the chemoreception of molecules in olfaction. Similarly, if subjective perception under the representationalist thesis is somehow attributed to electromagnetic or other physical fields, such fields also occur in all physical systems. Thus, representation by the brain does not detract from the conclusion of the universality of perception.
There is thus no reason to believe there is a threshold at which the subjective properties of perception in nature suddenly vanish and only the objective features remain. Obviously, it is hard to imagine what the subjective feel of perception at the most basic levels of interaction between molecules and particles might be like. The closest resemblance we will find is probably with the most basic of our own sensations - an odorous whiff, the pangs of hunger, an itch, a throb of pain or the urge of desire.
A reasonable, if not uncontentious, conclusion to be drawn from the foregoing discussion is that, considered both objectively and subjectively, neither a brain, neurons or sense organs are necessary for perception, and that an act of perception may occur in nature every time a causal interaction takes place.
Reference
Hickman, CP. Roberts, LS & Hickman, FM. 1984. Integrated Principles of Zoology. 7th ed. St Louis, Times Mirror/Cosby Publishing. A dated but still useful standard zoology text.
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